Also, the OM blackhawk...
is direct contact with primers, NM has transfer bar, (this allow full cylinder loading for NM whereas the OM MUST be loaded in FIVE chambers and carried on the EMPTY chamber of the cylinder.
Ruger DOES have a parts kit that converts the OM into a transfer bar system, it MUST be a factory install to keep timing the same.
Contact Ruger for more info. (Might have a nominal charge.)